Kinetic and spectral investigation of the electron and hydrogen adducts of dihydroxy- and dimethyl-substituted pyrimidines: a pulse radiolysis and product analysis study

Abstract

The reactions of hydrated electrons (e_aq^-) and hydrogen atoms (H·) with 4,6-dihydroxy-2-methylpyrimidine (DHMP), 2,4-dimethyl-6-hydroxypyrimidine (DMHP), 5,6-dimethyluracil (DMU) and 6-methyluracil (MU) were studied at different pH values using pulse radiolysis. The second-order rate constants obtained for the reaction of e_aq^- with these systems are in the range (5-10) × 10⁹ dm³ mol^-1 s^-1 at near, neutral pH. At basic pH, the rate constant values were considerably reduced owing to the electrostatic effect between e_aq^- and pyrimidine anion. The transient absorption spectra of the electron adducts have distinct absorption maxima at around 300-320 nm. The initial spectrum in the case of DHMP at pH 4.5 was found to undergo a first-order transformation. Based on the spectral characteristics and the yields of methylviologen radical cation (MV^•+) resulted from the electron transfer reaction between the electron adducts and MV²⁺, it is proposed that a protonated (at oxygen) electron adduct of DHMP is initially formed which undergoes a proton- and phosphate-catalysed transformation to form a reducing C(5) protonated C(6)-yl radical. Such preferential protonation at C(5) is predominant only with dihydroxypyrimidine systems. At pH 9 and 13, formation of a radical monoanion of DHMP (pK_a ≥ 13) is proposed. The possible attack of eaq- is proposed to be at N(1) or N(3) of DMHP. The resulting electron adduct has a pK_a value around 6.0. Similar properties for the electron adducts of DMU and MU [electron attack at O(4)] are proposed. The second-order rate constants for H· with DHMP, DMHP, DMU and MU were in the range (1.7-28) × 10⁸ dm³ mol^-1 s^-1. The hydrogen adduct spectra were generally identified as their absorption maxima at 310-380 and 460-510 nm. Formation of C(5)-protonated C(6)-yl radical, the same radical that formed after the H⁺-and phosphate-catalysed transformation of the electron adduct, is proposed for DHMP. The possibility of the formation of C(5)-yl and C(6)-yl H adducts of DMHP, DMU and MU is discussed. High-performance liquid chromatography coupled with electrospray mass spectrometry (HPLC-ES-MS) has been used to qualitatively analyse the products obtained from the reaction of e_aq^- with DHMP and DMHP and the results revealed that the products are mainly derived from the C(5) protonated C(6)-yl radicals via its disproportionation and dimerization reaction. A possible reaction mechanism is proposed for the product formation.